Writing Scala Compiler Plugins[3] This interesting tutorial, written by Lex Spoon, describes the mechanism by which the Scala compiler can be extended by plugging in additional modules that implement user-defined features and extensions to Scala. Contains a working sample plugin and instructions.

Scala Improvement Documents[4] The Scala Improvement Documents (SIDs) are technical documents that describe the implementation and usage of specific Scala features or components. A SID is usually written when a new feature is added to Scala, either after or during the development process, and acts as a manual and guide to the new development. You can read more information about SIDs[4], or browse the SID Library online[5].

Compiler Internals[6]A collection of video recordings of code walk-through sessions by Martin Odersky about the Scala compiler internals. The video quality is suboptimal, but the movies are a good reference to some of the most obscure details of the compiler architecture.